During the period of support of this work by HEW we have by use of model systems demonstrated the feasibility and the high precision obtainable with, our new method for the determination of the locations of hydrogen atoms in molecules in crystals. We have in addition applied this technique to the enzyme, hen egg white lysozyme, and shown clearly its ability to give detailed information concerning molecular structure in the vicinity of coordinated metal ions. We propose a continuation of this work with extension to a variety of problems relative to protein structures and with especial attention to proton locations at the active sites of enzymes. Such information, which is not available for enzymes from x-ray diffraction, will provide a very useful tool for determining local structure, for measuring changes in conformation of enzyme and substrate which are important for enzymatic action, and for determining the role of bound metals and of hydrogen bonds in enzymatic processes. The development of this method in the case of a molecule such as hen egg white lysozyme for which high resolution X-ray results have accurately fixed the positions of the heavier atoms, will permit its extension to other cases such as human lysozyme and other enzymes for which precise X-ray data are not available. This will then enable one to map the structure of active sites, observe the conformation of bound substrates, and study the enzymatic reaction mechanisms in many physiologically important cases. This new method employs electron paramagnetic resonance and electron nuclear double resonance absorptions of rare earth and other paramagnetic metal ions, bound to proteins in crystals. The anisotropies of the proton-electron hyperfine interactions measured by electron nuclear double resonance yield value of proton coordinates.